Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 041151

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
551 AM CST Sun Dec 4 2016

324 AM CST

Through tonight...

Main forecast concerns/challenges are with the expected
accumulating snow over much of northern Illinois and northwest
Indiana this morning into the late afternoon. Main forecast
changes this morning were to increase snowfall amounts and expand
the advisory southeast to now include Livingston/Kankakee counties
in Illinois and Lake/Porter counties in northwest Indiana.

Everything still appears to be on track for the first
accumulating snowfall today. Upper level trough noted on satellite
imagery, with several embedded impulses quickly lifting northeast
into the mid Mississippi Valley. Ahead of this feature, isentropic
ascent is also helping to saturate the column. Although returns
noted on radar, this saturation has not occurred quite yet, with
snow not occurring at this time. Adjusted pops in the near term to
reflect latest trends with upper level support providing the onset
of snow across the far western CWA right around 12Z. This would
include all areas in the west, along the I-39 corridor. Remaining
areas to the east in northern Illinois still on track for right
around a 15z time frame, and then soon there after for areas in
northwest Indiana. Given this noted time line, have adjusted
timing of the advisory, mainly to delay the eastern half until

As this upper level trough swings through the area later this
morning into the afternoon, expect a transition from light snow to
more moderate intensity snow as large scale ascent really ramps up
ahead of this feature. Most guidance in agreement with the
increase in forcing due to many features coming together, which
include a stout negatively tilted trough and more potential for
instability aloft. Would anticipate many locations across much of
northern Illinois and northwest Indiana to see this ramp up in
snow intensity by midday, and even more so by early to mid
afternoon. This not only includes areas along the I-39 corridor
which was previously noted, but now also includes many areas in
the northern two thirds of the CWA likely centered from just north
of I-88 south to along/south of I-80. In these locations, strong
forcing and more efficient snow production will likely allow for
an increase in snow amounts even into the afternoon when the more
intense snow will be able to offset melting. In the height of the
more intense snow, 18-23Z, am a little concerned that the snow
will come down hard enough to accumulate more on the roadways with
at least slushy conditions possible. All of this and the increase
in snow amounts, feel the advisory is still warranted and worthy
of expansion. Now have snow amounts of 3-5 inches along the I-39
corridor, 2-4 inches elsewhere in the advisory in northern
Illinois and northwest Indiana, and then 1-3 inches across the far
southeast CWA. In this location, rain may briefly mix with snow
but do think forcing will be strong enough even in this location
to help the column/boundary layer to cool to support all snow in
the afternoon. Snow will quickly taper off right around the 00z
time and although some light snow may still be ongoing across the
eastern CWA in the 00-03z period, it will likely be light enough
to not warrant extension of the advisory.



347 AM CST

Monday through Saturday...

Monday appearing dry/quiet on the backside of this system with
rising heights and building high pressure. Continued WAA will
usher in a warmer airmass but wonder what impact this will have on
high temps with cloud cover possibly lingering and with a snow
pack likely in place. Still keeping an eye on two possible systems
to affect the area during the week. Confidence is still on the
lower side with the first system arriving Monday night into
Tuesday. Timing and placement adding to the low confidence as well
as extent of the airmass which would be in place. Have continued
to limit pops as well as continued to mention both snow and rain.
Much colder airmass still expected later in the week, however,
confidence also remains low with snow potential Wednesday night
into Thursday. Still remains a possibility for snow but with
varying guidance, no change in confidence. Much colder air and
lake effect snow potential will then be concerns through the
remainder of the work week, as well as snow potential returning
CWA wide by the weekend.



For the 12Z TAFs...

Snow is set to impact the terminals today with a period of
moderate to heavy snow. Model agreement is still fairly good at
this point in terms of timing of snow onset and peak snowfall such
that the trends in the TAF are overall fairly good, with minor
tweaks in timing plausible. Expect light snow and MVFR vsbys at
onset to quickly transition to IFR vsbys. LIFR vsbys and even
periods of LIFR cigs are expected in the heavier snow. Model time
height forecasts depict a narrow snow growth region but strong
omega in that region ahead of a potent shortwave. As this wave
approaches, later this morning and early afternoon, this will
combine with impressive isentropic lift to bring a period of
moderate to heavy snowfall rates. Feel that 3/4-1SM vsby is an
accurate representation given that we do not have a deeper snow
growth region, but there is enough of signal to support at least
occasional 3/4sm vsby in the prime period of 18z-23z this
afternoon. Several guidance sources bring 1/2 mile vsbys in for a
time as well, but still some mixed signals as to how long this
will be. Therefore have held onto the 3/4sm but went prevailing as
several sources suggest pretty high confidence in sub 1sm. It is
plausible during the peak period we have 3/4 tempo 1/2 or just
prevailing 1/2 but will let upcoming amendments monitor trends
before hitting it that hard. Also tried to tighten up the timing
of vsby reduction and cig changes corresponding with the latest
high res expect snow onset times.

Snow will generally be of a wet nature with ratios of 10-12:1. It
is always challenging to tell the impacts of the snow on warm
runways this early in the season, but with 2-3" inches or so forecast
in many locations including the urban metro and higher amounts
west, expect that there will be some slushy accums on pavements as
well in the heavier snowfall periods. Winds will be out of the
southeast and generally on the lighter side, transitioning to
southwest as the snow tapers from west to east this evening.
Guidance is mixed on clearing overnight, expected that either it
does not occur or just for a time behind the snowfall and then
fills back in as we warm advect over the snow cover.




301 AM CST

Somewhat benign conditions will be found across the lake to start
the week, with gusty west-northwest taking hold for the latter

High pressure across the Great Lakes and Ohio Valley will move
east today. Meanwhile, a surface pressure trough across the Upper
Midwest will spread east across northern Lake Michigan tonight
into Monday. Another low pressure system will move into the upper
Midwest on Monday as weak high pressure spreads northeast through
the Ohio Valley. Quickly on the heels of the high, another low
will move in a similar fashion from the southern plains to the
Ohio Valley Tuesday. The result of these pressure patterns will be
a shift to west winds tonight and then a return to southwest
Monday and a portion of Tuesday.

Things get more active mid to late week as strong high pressure
will move into the northern plains Wednesday and into the nation`s
heartland by early Friday, while low pressure across Ontario will
shift east through Canada and merge with low pressure off the
Canadian maritimes by the weekend. These features allow a strong
cold front to shift across the lake late Tuesday and continue
under gusty wnw winds through at least Friday. Winds to 30 kt
seem a lock, and given the strong cold advection mixing should not
be a problem, but model winds above the surface are not much
higher. Will continue with a mention of gales in the GLF, with the
most likely period late Thursday into early Friday.



IL...Winter Weather Advisory...ILZ003-ILZ004-ILZ008-ILZ010-ILZ011-
     ILZ019-ILZ032 UNTIL 6 PM Sunday.

     Winter Weather Advisory...ILZ005-ILZ006-ILZ012-ILZ013-ILZ014-
     ILZ020-ILZ021-ILZ022-ILZ023...9 AM Sunday TO 6 PM Sunday.

IN...Winter Weather Advisory...INZ001-INZ002...9 AM Sunday TO 6 PM




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